The secrecy capacity of relay communications is investigated in this paper, where an eavesdropper is present. Distributed precoding through multiple relay nodes can be used to enhance the received signal power at the destination node and to mitigate the signal leakage to the eavesdropper, simultaneously. Due to individual power constraints at relay nodes, the distributed precoding scalar at each relay node is equivalent to a distributed precoding angle. An iterative algorithm is proposed to find the suboptimum distributed precoding angles at all the relay nodes, where the channel state information (CSI) sharing overhead among the relay nodes can also be substantially reduced. Each relay node receives the equivalent CSI from its preceding relay node, computes its distributed precoding angle, and updates the equivalent CSI for the next relay node. Compared with the simple decode-and-forward relaying protocol with random distributed precoding, the proposed iterative distributed precoding algorithm is able to further improve on the secrecy capacity of the multirelay wiretap system with an acceptable CSI sharing overhead.

This study investigates the security of the two‐way relaying system with untrusty relay nodes. Cooperative jamming schemes are considered for bi‐directional secrecy communications. The transmit power of each source node is divided into two parts corresponding to the user and jamming signals, respectively. Two different assumptions of the jamming signals are considered. When the jamming signals are a priori known at the two source nodes, closed‐form power allocation expressions at two source nodes are derived. Under the assumption of unknown jamming signals, it is proven that the cooperative jamming is useless for the system secrecy capacity, because that all the power should be allocated to the user signals at each source node. Relay selection is also investigated based on the analysis of cooperative jamming. Simulation results are presented to compare the system secrecy capacities under the two jamming signal assumptions.

Carrier aggregation is one of the key techniques for the advancement of long-term evolution (LTE- Advanced) networks. This article proposes a quality- of-experience (QoE)-based carrier scheduling scheme for networks with multiple services. The proposed scheme aims at maximizing the user QoE, which is determined by both the application-level and network-level quality of services. Packet delay, as an essential factor affecting QoE, is first discussed under the context of QoE optimization as well as the data rate. The component carriers are dynamically scheduled according to the network traffic load by the proposed novel scheme. Simulation results show that our approach can achieve significant improvement in QoE and fairness over conventional approaches.

In hybrid automatic repeat request (HARQ), maximum ratio combining (MRC) is usually employed at the receiver if the retransmission signal is the same as that in the first transmission. However, conventional performance analysis of HARQ and MRC does not consider that all previous transmissions fail. In this paper, we analyze the conditional error rate of HARQ with arbitrary combining schemes. Both the theoretical and approximate expressions of the conditional error rate are derived. Our analytical and simulation results reveal that conventional MRC is not optimum for HARQ, especially when the packet is short or the channel gain of the first transmission is much better than that of retransmission.

In wireless sensor networks, several relay nodes can be used to support multiple transmission links between source and destination nodes simultaneously to achieve higher spectral efficiency. A channel state information sharing scheme among multiple relay nodes is presented in this letter, where only one angle per source-destination pair is sent from one relay node to another. Then, to enhance received signal powers, a distributed multiuser precoding scheme is presented to be used at each relay node with the angles sent by another relay node. Simulation results demonstrate that the proposed distributed precoding scheme enhances the capacity performance with slight overhead.

The use of Unmanned Arial Systems (UAS) in both civil and military applications has greatly increased over the last few years. At the same time, issues related to the safety of air traffic operation arise when considering UAS insertion into non-segregated airspace. Thus, surveillance plays a key role in monitoring and controlling air traffic, enabling UASs and manned aircrafts to insure themselves against collision. Automatic Dependent Surveillance-Broadcast (ADS-B) has been proposed as a potential solution. It empowers aircraft to automatically broadcast their locations and intents to provide enhanced situational awareness. However, the use of two different ADS-B frequencies (1090MHz or 978MHz)restricts communication between different types of aircraft mainly relating to their routine flight altitude. Aircraft with ADS-B capability is either through 978 MHz Universal Access Transceivers (UAT) on general aviation below 18,000 ft or 1090 MHz Extended Squitter (1090ES) transponders above on commercial flight. In view of the manoeuvrability of UAS with a wide flight range, it will bring safety hazards and risks related to surveillance systems, if a single-frequency ADSBis adopted in a traditional way. Motivated by this, this paper proposes a novel Software-Defined Radio (SDR)-basedADS-B with dual-frequency for UAS that is called SAS. It exploits the SDR technology that uses software running on a generic hardware platform to perform signal processing. The SAS can thus integrate the dual-datalink hardware devices into a single generic device, which improves flexibility and connectivity of surveillance for general use. We performed simulation studies with SDR-based BeeCube and also built its prototype implementation to verify its performance.